(1) Field of the Invention
The present invention relates to a curing agent for powder coatings, a powder coating composition comprising said curing agent, and powder coating film. More particularlly, the present invention relates to a curing agent for powder coatings which comprises a highly reactive carbodiimide compound, the powder coating composition which comprises said curing agent, and can be baked at low temperatures and gives no adverse effect on the environment where this composition is applied as powder coating film.
(2) Related Art
As environmental problems become increasingly serious, coatings used in the coating industry are being rapidly changed from solvent type coatings to aqueous coatings, high-solid coatings (in which the amount of solvent is reduced in such an extent that the operability or the properties of the formed film are not impaired), electron beam- or ultraviolet-curing coatings, and powder coatings. Of them, the aqueous coatings, however, have various problems as follows. They cause rusting of applicator, dryer, etc. because they use water as a diluent; they need a large energy for drying of the water used as a diluent and are not economical; and the coating film formed has low water resistance because the resin used therein is hydrophilic.
The high-solid coatings, which use a very small amount of a solvent or no solvent and has a very high solution viscosity, are difficult to make into a thin film and therefore cannot be applied by an ordinary applicator. The electron beam-or ultraviolet-curing coatings require, at the curing zone, a special apparatus capable of applying a light, in place of a heating type dryer, and therefore need a large investment.
Under such a situation, the powder coatings, which are one of the next-generation coatings, are drawing the highest attention, and various application methods therefor have been developed such as electrostatic application, fluidized dipping, spraying and the like. These application methods have various advantages. That is, no odor or no hazardous substance is generated because of no use of solvent; the loss of coating is small; the recovery of oversprayed coating is easy, resulting in higher economy; no complicated application technique is needed, making possible automated, rational application.
Moreover, the powder coatings are superior in abrasion resistance, chemical resistance, acid resistance, water resistance, solvent resistance and impact resistance; therefore, they are finding wide usages in coating of automobiles, electrical appliances, cladding panels, guard rails, furniture, etc.
Conventional powder coatings, however, have had considerable restriction in utilization because they need very high baking temperatures of 160 to 230.degree. C., and the high baking temperature has necessitated use of a substrate of high heat resistance.
Conventional powder coatings have further had a problem when they are a polyester/urethane type coating. That is, since this type uses a blocked isocyanate as a curing agent, the coating generates, during baking, a harmful volatile organic compound (VOC) derived from the curing agent and the compound poses an odor problem.
The present invention has been completed in order to alleviate the above-mentioned problems of conventional powder coatings and provide:
a powder coating composition which retains various properties (e.g. excellent curability) possessed by ordinary powder coatings; can be baked at low temperatures and therefore can be applied with conventional equipment used in application of solvent type coatings; generates no harmful volatile organic compound during the baking or heating; can be applied even onto a substrate of low heat resistance or onto a substrate which has too large a heat capacity and which is difficult to heat to a desired application temperature when used for conventional powder coatings; and is low in running cost or maintenance cost; PA1 a curing agent used in the above powder coating composition; and PA1 powder coating film. PA1 (A) a curing agent for powder coatings, which comprises, as a main component, a carbodiimide compound represented by the following general formula (1), (2) or (3): EQU OCN--R.sub.1 --(NCN--R.sub.2).sub.n1 --NCO (1) PA1 (B) a powder coating composition comprising: PA1 (C) powder coating film, which is obtained by reacting and curing at least one kind of the above curing agent (A) for powder coatings, and a bifunctional resin compound having a functional group reactive with carbodiimide group. PA1 catalyst amount: 0.1 to 10% by weight, preferably 0.5 to 5% by weight, PA1 reaction temperature: 120 to 200.degree. C., and when an aromatic diisocyanate is carbodiimidized, the following conditions are employed: PA1 catalyst amount: 0.01 to 5% by weight, preferably 0.05 to 1% by weight, PA1 reaction temperature: 50 to 180.degree. C.